1. Field of the Invention
The present invention relates to a hydroprocessing catalyst and its use in the hydroprocessing of heavy hydrocarbon oils.
2. Prior Art
Hydrocarbon oils containing 70 wt. % or more of components boiling above 450° C., in particular hydrocarbon oils containing 50 wt. % or more of components with a boiling point of 538° C. or higher, are called heavy hydrocarbon oils. These include atmospheric residue (AR) and vacuum residue (VR), which are produced in petroleum refining. It is desired to remove impurities such as sulfur from these heavy hydrocarbon oils by hydroprocessing, and/or to convert them into lighter oils, which have a higher economic value. Depending on the properties of the feed, this is advantageously done in fixed bed or in ebullating bed operation.
Various catalysts have been proposed for this purpose in the art. Generally, these catalysts are capable of removing sulfur, Conradson carbon residue (CCR), various metals, nitrogen and/or asphaltenes. However, it was found that the decomposition of asphaltenes, which are aggregates of condensed aromatic compounds, is generally accompanied by the formation of sediment and sludge. Sediment can be determined by the Shell hot filtration solid test (SHFST) (see Van Kerknoort et al., J. Inst. Pet., 37, 596 604 (1951)). Its ordinary content is said to be about 0.19 to 1 wt. % in product with a boiling point of 340° C. or higher collected from the bottom of a flash drum.
Sediment formed during ebullating bed hydroprocessing may settle and deposit in such apparatuses as heat exchangers and reactors, and because it threatens to close off the passage, it may seriously hamper the operation of these apparatuses.
Japanese Patent Laid-Open No. 1994-88081 discloses a hydroprocessing method for heavy hydrocarbon oils using a catalyst with a specific pore size distribution. In this method a catalyst is used with 3 to 6 wt. % of a Group VIII metal oxide, 4.5 to 24 wt. % of a Group VIB metal oxide, and 0 to 6 wt. % of phosphorus oxides loaded onto a porous alumina carrier which has a specific surface area of 165 to 230 m2/g, a total pore volume of 0.5 to 0.8 ml/g, and a pore size distribution wherein 5% or less of the total pore volume is present in pores with a diameter less than 80 Å, 65–70% of the total pore volume present in pores with a diameter below 250 Å is present in a range of 20 Å below the MPD to 20 Å above the MPD, and 22–29% of the total pore volume is present in pores with a diameter of more than 250 Å.
However, although this method can achieve efficient hydrodesulfurization and Conradson carbon reduction, it does not solve the problem of sediment formation.
Japanese Patent Laid-Open No. 1994-200261 discloses a hydroprocessing method for heavy oils, and a catalyst used to implement this method. In this reference a catalyst was proposed with 2.2 to 6 wt. % of a Group VIII metal oxide and 7 to 24 wt. % of a Group VIB metal oxide on a porous alumina carrier, which catalyst has a surface area of 150–240 m2/g, a total pore volume of 0.7 to 0.98 ml/g, and a pore size distribution wherein less than 20% of the total pore volume is present in pores with a diameter of less than 100 Å, at least 34% of the total pore volume is present in pores with a diameter of 100–200 Å, and 26–46% of the total pore volume is present in pores with a diameter of more than 200 Å. However, the present inventors have found that this catalyst shows a too high sediment formation.
Japanese patent publication 2-48485 describes a process for preparing an alumina catalyst carrier which has 0.6 to 0.85 ml/g of its pore volume in pores with a diameter below 500 Å and 0.1 to 0.3 ml/g of pore volume in pores with a diameter of 1,000 to 10,000 Å. The pore mode in the range up to 500 Å is 90–210 Å. The U-value, defined as D50/(D95-D5), is at least 0.55. The macropore volume of this carrier is very high, making it difficult to maintain stable hydrodesulfurization activity.
U.S. Pat. No. 4,395,329 describes a hydroprocessing catalyst for heavy oils which has a specific pore size distribution. The catalysts described in this reference have 10–25% of pore volume present in pores with a diameter above 10,000 Å. Especially when these catalysts are made by extrusion, this will detrimentally affect the strength of the catalyst and it is expected that it will be difficult to use the catalyst commercially.
As indicated above, in the improvement of ebullating bed hydroprocessing catalysts there is need for a catalyst which achieves a high level of contaminant removal, in particular metals and asphaltene removal, with low sediment formation and high conversion of the fraction boiling above 538° C.
In fixed bed operation, sediment formation is not much of a problem. However, also in fixed bed operation there is need for a catalyst which shows improved asphaltene removal and hydrodemetallization activity. Additionally, it would be attractive to have a catalyst which combines asphaltene removal with limited resin hydrogenation, because this leads to an improved storage stability of the product produced.
It has been found that these problems can be solved by the provision of a catalyst comprising a Group VI metal and a Group VIII metal on a carrier comprising alumina, which catalyst has a specific pore size distribution, including a limited pore volume in pores with a diameter above 4,000 Å, and a relatively high pore volume in pores with a diameter of 100–1,200 Å.